Electrical lighting represents about 20% of electricity use in the United States, including electrical power consumed for light generation and for space-cooling to accommodate light bulb heat gains from lighting inefficiencies, with commercial buildings using more interior space lighting than all other sectors combined. A great majority of interior space lighting in commercial buildings occurs during daylight hours, and the potential for using solar interior lighting for such applications has long been recognized. One approach for solar interior lighting is the use of sky lights, essentially windows in a roof. That approach, however, requires direct solar collection into the space to be lighted, essentially limiting the technique to single-story buildings. A second approach is to collect solar light and conduct the light to an interior space where the light is needed. Light wells and internally-reflective light ducts represent simple light conduction techniques, but are challenging to route, consume large volumes of building space and interfere with building architecture. Attempts have been made to concentrate collected solar light by several hundred times or more and transmit the concentrated light to interior spaces in optical fibers. One approach has been to concentrate the light using a concentrating mirror and feed the concentrated light into optical fibers. A significant problem with such an approach is the potential for significant heat generation in equipment used to concentrate the light and to direct the concentrated light into the optical fibers, and especially due to the infrared radiation component of collected solar light.
A technique stemming from work at Oak Ridge National Laboratories reduces thermal problems by removing infrared radiation from the concentrated light prior to introduction of the concentrated light into the optical fibers. This technique significantly improves thermal performance, but even with the removal of a great majority of the infrared radiation, it has proved difficult to provide systems that provide highly concentrated solar radiation for interior lighting applications at reasonable cost. Potential for component overheating remains a significant problem that has been addressed to different degrees through the use of expensive equipment (e.g., through use of extremely precise solar trackers and/or high-cost glass optical fibers) to operate at very high light concentration factors of 1000 or more or through operation at much lower light concentration factors but using lower-cost components (e.g., using less precise solar trackers and/or plastic optical fibers) and installed systems tend to be highly customized for each specific application and not easily adapted to other situations. Practical, robust, cost-effective solar interior lighting solutions applicable for a wide variety of interior lighting situations remain elusive.